The present invention relates generally to a stencil wiping device. More specifically, the present invention relates to devices for cleaning excess solder paste from the bottom (contact) side of printed circuit board stencils.
In the manufacture of a circuit boards, conductive ink or solder paste is selectively printed on a substrate such as a ceramic substrate or a fiberglass circuit board. A metal or plastic stencil is often used for this printing operation and particularly where solder bumps are printed on the substrate. In prior operations of this type, the circuit board to receive solder bumps is placed on a vacuum printing nest. The nest is attached to a slide which, following alignment, is shuttled under a metal stencil which is preferably made of brass or stainless steel. Once under the stencil, the stencil moves down to make contact with the circuit board and solder paste is deposited on the stencil and is squeezed through the holes in the stencil by a squeegee which is moved across the upper face of the stencil. When the printing is complete, the stencil is lifted off of the circuit board, and the circuit board, nest and slide, being connected together, are shuttled back to an operator where the circuit board is removed and the process is repeated.
However, excessive solder often gathers on the edges of the holes of the stencil and impedes the flow of solder to the circuit board. Further, solder paste or conductive ink may wick along the bottom face of the stencil adjacent the hole formed therein. This results in smudged conductive lines or solder paste upon subsequent printing and may even result in unintentional shorting between circuit pads. Further, the solder bumps printed in this manner are notably deformed and hence are not of uniform quality from board to board. This may cause further problems when a semiconductor device, such as an integrated flip-chip, is connected and secured to the solder deposition.
Operators have been known to manually wipe the bottom of the solder paste stencil to remove excess solder paste. However, this is not sufficient to dislodge solder gathered in the holes of the stencil. Further, manual removal of excessive solder paste is messy, labor and time intensive, and not always reliable.
There have been many attempts in the prior art to address the foregoing concerns. For example, FIG. 1 shows a prior art apparatus 2 for wiping the bottom surface of a stencil 14. A lint-free cleaning sheet 1 is fed from a supply roll 3 and through a set of tensioning rollers 5. Solvent is applied to the sheet 1 by a solvent bar 11. The solvent is pumped into the solvent bar 11 by a pump (not shown) and the liquid from the solvent bar 11 through porous walls onto the sheet 1. The sheet 1 passes over a pair of flexible blades 13 which press the sheet 1 against the stencil 14. A vacuum channel 15 supports the blades 13.
Suction is optionally applied to the vacuum channel 15 to pull the stencil 14 tightly against the sheet 1 as vacuum is applied from the channel 15 through an outlet tube 17. The sheet 1 then passes through a second set of tensioning rollers 19 and is taken up on a take-up roll 21. Cross members 16 are disposed across the width of the vacuum channel 15.
FIG. 2 shows a top view of the blades 13 and vacuum channel 15 shown in prior art FIG. 1. The cross section of FIG. 1 is taken along line A in FIG. 2. A number of cross members 16 are disposed across the vacuum channel 15.
To wipe the stencil 14 the sheet 1 is advanced past the solvent bar 11 to apply solvent to the portion of the sheet 1 which will be positioned between the blades 13 and the stencil 14. The apparatus 2 is then moved as shown by arrows 23 so that the blades 13; wipe the solvent-laden sheet 1 across the stencil 14. Optionally, suction applied via the vacuum channel 15 increases the force between the sheet 1 and the stencil 14.
The use of solvent in the above apparatus facilitates the removal of debris, particularly dried solder paste. Without the use of the solvent, wiping using the above apparatus is unreliable. The use of solvents, however, presents a number of problems in the manufacturing process. Many solvents contribute to environmental pollution and are hazardous to workers. Certain solvent vapors can cause smog or contribute to ozone destruction in the upper atmosphere. Many solvents are toxic and/or highly flammable. As such, their use is heavily regulated and regulatory compliance increases the cost of manufacturing circuit boards. Less hazardous solvents are available but they are expensive and less effective in removing dried solder paste. The paper rolls are contaminated with solder paste that has a high lead content. Disposal of such waste is very costly due to government regulations.
The prior art apparatus described with reference to FIG. 1 can be used without applying a solvent to the sheet. The result, however, is that cleaning of the stencil is less effective and a greater number of unreliable solder joints and shorts are formed on the finished printed circuit board.
The amount of solvent that is required for the process can be reduced with acceptable reliability if workers periodically wipe the stencil by hand using a solvent soaked cloth instead of applying solvent to the automated wiping apparatus shown in prior art FIG. 1. This procedure presents other difficulties, however. Hand wiping presents a health risk to workers by increasing their exposure to metals, particularly lead, in the solder paste. In addition, hand wiping requires that the automated machinery be stopped so that a worker can reach into the apparatus. Stopping the screen printer to wipe the stencil may result in idling equipment used in subsequent processing steps thus decreasing the productivity of the production line.
In view of the foregoing, there is a need for an automated, reliable method of cleaning solder paste and conductive ink stencils and removing solder paste or ink gathered in the apertures and, in particular, on the bottom side on the edges of the apertures of the stencil. There is also a demand for a stencil wiping device that is less costly to operate that prior art paper roll-based cleaning stencil cleaning systems. Further, there is a demand for a stencil wiping device that is fast in order to reduce down time of the stencil while providing superior solder paste removal. There is still a further demand for a stencil wiping device to enable the reclaiming of removed solder paste for recycling thereof.
The present invention preserves the advantages of prior art devices for cleaning solder paste from printed circuit board stencils. In addition, it provides new advantages not found such currently available cleaning devices and overcomes many disadvantages of such currently available devices.
The stencil cleaning apparatus, in accordance with the present invention, includes a container of cleaning solution therein and a blade holder that is movable between a wiping position and the container of cleaning solution. A wiping blade or multiple blades are mounted in the blade holder. The blade holder, with the blade mounted thereon, is reciprocated back and forth when in communication with a stencil to be cleaned. The blade is moved from the wiping position in communication with the stencil into the cleaning solution in the container. A pneumatic piston and rotary actuator provides controlled movement of the wiping blade. The wiping blade may be vibrated during wiping to improve removal of solder paste from the stencil and the cleaning solution may be ultrasonically vibrated at high frequency to improve removal of solder paste from the wiping blade. The wiping blade may also be pulsed into a sponge to remove cleaning excess solution and semi-dry the blades prior to the next cleaning cycle.
In accordance with the method of the present invention, a cleaning bath of cleaning solution and a wiping blade are provided. The wiping blade is reciprocating while it is in contact with a stencil to remove excess solder paste therefrom. The wiping blade is then rotated 180 degrees into a cleaning bath which is, preferably ultrasonically vibrated, to remove the collected excess solder paste from the stencil. The clean wiping blade is removed from the cleaning bath and is then, preferably, pulsed into a sponge to remove any excess cleaning solution remaining on the wiping blade. An air tube may also be provided to blow air through the stencil from the top and into communication with the wiping blade to improve solder paste removal as well as containing solder paste particles that were airborne.
It is therefore an object of the present to provide a stencil wiping device that is more efficient and faster than prior art stencil cleaning devices.
Another object of the present invention is to provide a stencil wiping device that is less expensive to operate than prior art stencil cleaning devices.
It is a further object of the present invention to provide a stencil wiping device that can effectively re-claim solder paste cleaned from the bottom side of a printed circuit board stencil.
It is a further object of the present invention to provide a stencil wiping device that is 100 percent effective.